You will make fun of - or mock the person with an imitation of them. In short, you make fun of them by pretending to be them. This is often a famous person so that almost anyone will recognize what is being done. Some people are famous for imitating other famous people. For instance, Jim Carry began his career with impressions.

to do an impression of someone is to act like someone else. take for example, Robin Williams. He does impressions of other people. For example, lets make up a friend. We'll call him Fred. He is into math, chess, and quadratic equations. lets say you on the other hand, are into sports, country music, and can never understand a word he says. If you were to do an impression of him you would change your voice to sound like him and say something like "Look at me, I'm Fred. 2+2 is 4 and you can't move your knight to E5 because that is physically impossible"